Recently, it has become more and more necessary to further enhance the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T (tesla) or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot have that high remanence Br of 0.5 T or more.
An Sm—Co based magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet with that high remanence Br of 0.5 T or more. Examples of other high-Br magnets include an Nd—Fe—B based sintered magnet produced by a powder metallurgical process and an Nd—Fe—B based quenched magnet produced by a melt quenching process. An Nd—Fe—B based sintered magnet of the former type is disclosed in Patent Document No. 1, for example, and an Nd—Fe—B based quenched magnet of the latter type is disclosed in Patent Document No. 2, for instance.
However, the Sm—Co based magnet is expensive, because its materials Sm and Co are both expensive.
As for the Nd—Fe—B based magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically accounting for about 60 wt % to about 70 wt % of the overall magnet), and is much less expensive than the Sm—Co based magnet. Nevertheless, it is still expensive to produce the Nd—Fe—B based magnet. This is partly because huge equipment and a great number of process steps are needed to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for about 10 at % to about 15 at % of the magnet. Also, a powder metallurgical process normally requires a relatively large number of process steps by its nature.
In contrast, the Nd—Fe—B based quenched magnet produced by a melt quenching process can be obtained through relatively simple process steps of melting, melt quenching and heat-treating. Thus, compared to an Nd—Fe—B based magnet formed by a powder metallurgical process, an Nd—Fe—B based quenched magnet can be produced at a lower process cost. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Nd—Fe—B based quenched magnet produced by a melt quenching process has a lower Br than an anisotropic Nd—Fe—B based sintered magnet produced by a powder metallurgical process.
As disclosed in Patent Document No. 3, a technique of adding at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr in combination effectively improves the properties of an Nd—Fe—B based quenched magnet. When these elements are added, the magnet can have its coercivity HcJ and anticorrosiveness increased. However, the only known effective technique of improving the remanence Br is increasing the density of a bonded magnet. Also, if the Nd—Fe—B based quenched magnet includes at least 6 at % of rare-earth element, a melt spinning process, in which a melt is ejected through a nozzle against a chill roller, has often been used in the prior art to quench the melt at an increased rate.
As for an Nd—Fe—B based quenched magnet, an alternative magnet material was proposed in Non-Patent Document No. 1. The magnet material has a composition including a rare-earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages); and an Fe3B type compound phase as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes are distributed finely and uniformly as a composite of these two crystalline phases. Therefore, a magnet made from such a material is called a “nanocomposite magnet”. It was reported that such a nanocomposite magnet has a remanence Br as high as 1 T or more. But the coercivity HcJ thereof is relatively low, i.e., in the range of 160 kA/m to 240 kA/m. Accordingly, this permanent magnet material is applicable only when the operating point of the magnet is 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See Patent Documents Nos. 4, 5, 6 and 7, for example. However, none of these proposed techniques are reliable enough to always realize a sufficient “characteristic value per cost”. More specifically, none of the nanocomposite magnets produced by these techniques realizes a coercivity that is high enough to actually use it in various applications. Thus, none of these magnets can exhibit reasonably good magnetic properties in practice.
Also, a technique of obtaining Nd2Fe14B and α-Fe phases with grain sizes on the order of several tens nm was reported. According to this technique, an amorphous former La is added to a material alloy. Next, the melt of the material alloy is quenched to obtain a rapidly solidified alloy mainly composed of amorphous phases. And then the alloy is heated and crystallized to nucleate and grow both the Nd2Fe14B and α-Fe phases simultaneously. See Non-Patent Document No. 2. This article also teaches that adding a refractory metal element such as Ti in a very small amount (e.g., 2 at %) improves the magnet performance and that the mole fraction of Nd, rare-earth element, is preferably increased from 9.5 at % to 11.0 at % to reduce the grain sizes of the Nd2Fe14B and α-Fe phases. The refractory metal is added to reduce the nucleation of borides such as R2Fe23B3 and Fe3B and to make a magnet consisting essentially of Nd2Fe14B and α-Fe phases only. According to this technique, the rapidly solidified alloy to make a nanocomposite magnet is prepared by a melt spinning process in which a molten alloy is ejected through a nozzle onto the surface of a chill roller that is rotating at a high velocity. The melt spinning process can be used effectively to make an amorphous rapidly solidified alloy because a process of this type ensures an extremely high quenching rate.
In order to overcome these problems, the applicant of the present application developed a nanocomposite magnet, in which Ti is added to a composition range, including less than 10 at % of rare-earth element and more than 10 at % of boron, such that the nucleation of α-Fe is reduced during the melt-quenching process and that the volume percentage of a compound with an R2Fe14B-type crystal structure is increased, and disclosed in Patent Document No. 8.
Furthermore, Patent Documents Nos. 9 and 10 cite a number of elements, including Al, Si, V, Cr, Mn, Ga, Zr, Mb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb, which can be added to a nanocomposite magnet.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 59-46008        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 60-9852        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 1-7502        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 3-261104        Patent Document No. 5: Japanese Patent No. 2727505        Patent Document No. 6: Japanese Patent No. 2727506        Patent Document No. 7: PCT International Publication No. WO 00/03403        Patent Document No. 8: Japanese Patent Application Laid-Open Publication No. 2002-175908        Patent Document No. 9: Japanese Patent Application Laid-Open Publication No. 2002-285301        Patent Document No. 10: Japanese Patent No. 3297676        Non-Patent Document No. 1: R. Coehoorn et al., J. de Phys, C8, 1988, pp. 669-670        Non-Patent Document No. 2: W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of α-Fe/R2Fe14B-type Nanocomposites”, IEEE Trans. Magn. No. 5, INTERMAG. 99, Kyongiu, Korea, pp. 3265-3267, 1999.        